Social and economic capital as effect modifiers of the association between psychosocial stress and oral health

Objectives To assess the extent of the association of psychosocial stress with oral health in an Ontario population stratified by age groups, and whether any association is modified by indicators of social and economic capital. Methods We retrieved data of 21,320 Ontario adults, aged 30–74 years old, from the Canadian Community Health Survey (CCHS: 2017–2018), which is a Canada-wide, cross-sectional survey. Using binomial logistic regression models that adjusted for age, sex, education, and country of birth, we examined the association of psychosocial stress (indicated by perceived life stress) with inadequate oral health (indicated as having at least one of the following: bleeding gums, fair/poor self-perceived oral health, persistent oral pain). We assessed the effect measure modification of indicators of social (sense of belonging to the local community, living/family arrangements) and economic capital (household income, dental insurance, dwelling ownership) on the perceived life stress-oral health relationship, stratified by age (30–44, 45–59, 60–74 yrs). We then calculated the Relative Excess Risk due to Interaction (RERI) which indicates the risk that is above what would be expected if the combination of low capital (social or economic) and high psychosocial stress was entirely additive. Results Respondents with higher perceived life stress were at a significantly higher risk of having inadequate oral health (PR = 1.39; 95% CI: 1.34, 1.44). Adults with low social and economic capital were also at an increased risk of inadequate oral health. Effect measure modification showed an additive effect of indicators of social capital on the perceived life stress-oral health relationship. This effect was evident across all three age groups (30–44, 45–59, 60–74 yrs), with the highest attributable proportion of social and economic capital indicators in the psychosocial stress-oral health relationship in older adults (60–74 yrs). Conclusion Our findings suggest an exacerbating effect for low social and economic capital in the relationship of perceived life stress with inadequate oral health among older adults.


Introduction
Oral diseases continue to constitute a significant burden to individuals and societies. In addition to impacting the quality of life, oral diseases are strongly linked to major health conditions such as cardiovascular diseases and diabetes and have been shown to have cumbersome direct and indirect costs to societies and health care systems [1]. This burden is inequitably distributed amongst social groups of the population creating pervasive oral health inequalities. Over the past two decades, there has been a surge in accumulating research investigating various aspects of oral health inequalities, including their fundamental causes, the pathways involved in their production, and their impact at the individual and societal levels.
Psychosocial factors have been extensively investigated in relation to oral health and health inequalities [2][3][4][5]. Several studies have shown that the prolonged exposure to chronic psychosocial stress is associated with an increased risk of poor oral health in both children and adults [6]. Psychosocial stress has also been found to be associated with the development and progression of oral and systemic diseases and to partly explain the social inequalities in oral and general health outcomes [7][8][9]. Meanwhile, positive psychosocial exposures such as social support and perceptions of self-efficacy and mastery are known to contribute to better oral health outcomes, such as having more functional teeth and enhanced oral health-related quality of life [7]. A number of pathways have been shown to link psychosocial factors to oral health, including the behavioural and biological components of the stress pathway. For example, individuals exposed to chronic stress are more likely to engage in health-harming behaviours such as smoking and alcohol, potentially as coping mechanisms. In addition, psychosocial stress elicits a biological stress response which results in a cascade of pathobiological alterations leading to inflammation and subsequent oral diseases [5,10].
Meanwhile, theories from social epidemiology suggest that the relative social and economic positions of individuals and populations determine the magnitude of their exposure to psychosocial stress and its mitigation. The Commission on the Social Determinants of Health Framework [11] posits social and economic factors, such as income and education as well as the social support and social networks in which individuals are embedded on a daily basis, as fundamental causes of ill-health with psychosocial stress being as a strongly suggested pathway. Despite the large body of evidence on the links between structural, social, and economic factors with oral health, there appears to be little investigation on their possible buffering role on the impact of psychosocial stress on oral health. Investigating this is important for informing effective policies that aim to target oral health and oral health inequalities.
Social capital was described by Bordieu [12] and Coleman [13] as the social resources that are available to both an individual and the community through their social relationships that are characterized by mutual trust, such as social networks. Economic capital, on the other hand, refers to the material assets or resources that could be used to acquire or maintain better health, such as household income and health insurance [12,14]. Both social and economic capital can influence health outcomes in various ways. Social capital can protect and strengthen health by encouraging engagement in healthy behaviours, reinforcing psychological resources such as self-esteem, supplying emotional support, and decreasing exposure to stressors [15]. Meanwhile, the impact of economic capital on health outcomes is evident in the health inequalities observed with the inequitable distribution of material resources. These in turn can determine the probability of an individual encountering a health problem or experiencing financial stress [14].
The relationship between social capital and oral health at both the community and individual level have been well-documented, where higher social capital such as having strong social relationships have been shown to have a protective effect on oral health. In adults, social support has been suggested to be a determinant of increased oral health-related quality of life [16], and good social capital has been shown to be associated with a lower prevalence of root decay [17]. On the other hand, loneliness and social isolation have been consistently linked to the onset of periodontal disease [18,19]. Among older adults, low social capital has been shown to be associated with having fewer teeth [20] and increased periodontal attachment loss [21]. The relationship between economic capital and oral health is also well-investigated, and strongly supported. Most oral health outcomes such as oral health-related quality of life, dental caries and periodontal disease concentrate in individuals with lower household income, lower education, and those who lack dental insurance [22][23][24][25][26][27]. In adult clinical studies, higher income was shown to reduce the risk of periodontal disease [28], while the lack of financial support was associated with an increased number of missing and decayed teeth, as well as prevalence of oral pain in a Canadian population [24].
While there is a general consensus on the contribution of social and economic capital to oral health, their role in the relationship between psychosocial stress and oral health is less understood. We derive our hypotheses based on the buffering hypothesis described by Cohen and Wills [29] which postulates that the effect of psychosocial stress on health outcomes may be buffered, i.e. modified by levels of social support [30], and which we expand on by investigating the buffering or modifying role of social capital. As well, we build on previous research showing economic factors to modify the relationship between stress and health outcomes. Our objectives in this study were to quantify the extent to which psychosocial stress, measured using perceived life stress, is associated with oral health, and then determine the extent to which indicators of social (sense of belonging to the local community and living/family arrangements) and economic capital (household income, dental insurance, and dwelling ownership) can modify this relationship. Our hypothesis was that in lower social and economic capital individuals, there will be a stronger association between psychosocial stress and oral health.

Data source and study population
We used data from the Canadian Community Health Survey (CCHS) from the 2017-2018 cycle. CCHS is a cross-sectional survey conducted by Statistics Canada that collects information related to health status, health care utilization, and health determinants among Canadians aged 12 and older [31]. The survey excluded residents of Indian reserves, health care institutions, some remote areas, and full-time members of the Canadian Forces [31]. Data were collected using the CCHS questionnaire designed for computer assisted interviewing [31]. Both telephone and online electronic interviews were used to collect data, and each interview had an entry component, a health content, and an exit component. Each group of questions in the CCHS focused on a particular theme of health, such as general health, chronic conditions, smoking, and alcohol use. Some modules, such as dental visits and oral health, were part of the 2017-2018 cycle's optional content. Therefore, our analysis focused on adults in Ontario, Canada's most populated province, between the ages of 30 and 74 years old with complete data, for a total sample of 21,320, which represents a weighted sample of approximately 8,175,534 Ontario residents.

Variables
Exposure. Psychosocial stress. Psychosocial stress was measured in CCHS using singleitem Likert responses to the question "Thinking about the amount of stress in your life, would you say that most days are . . . not at all stressful, not very stressful, a bit stressful, quite a bit stressful, or extremely stressful?" The reliability and validity of single-item psychosocial stress measures compared to multiple item instruments has been established, showing their usefulness in large population-based epidemiological studies [32]. Given the ordinal level of measurement, we dichotomized this variable as "low stress" (not at all stressful, not very stressful, a bit stressful) and "high stress" (quite a bit stressful, extremely stressful).
Effect modifiers. Social capital. Indicators of social capital included two variables: sense of belonging to the local community, living/family arrangements, and household size. Sense of belonging to the local community, originally measured in CCHS on a 4-point scale from very strong to very weak, was dichotomized as "low sense of belonging" (very weak, somewhat weak) and "high sense of belonging" (very strong, somewhat strong). Living/family arrangements was characterized using a series of eight variables: unattached individual living alone; unattached individual living with others; individual living with spouse/partner; parent living with spouse/partner and child(ren); single parent living with children; child living with a single parent with or without siblings; child living with two parents with or without siblings; and other. We further dichotomized living/family arrangements as "living alone" (unattached individual living alone) and "living with others".
Economic capital. Indicators of economic capital included three variables: total household income, having dental insurance and dwelling ownership. In Ontario, the low-income cut-off is determined based on family size (i.e., number of persons living in one household) and community size (i.e., number of persons living in a community) [33]. In 2017 and 2018, a family size of three persons and a community size of 500,000 persons or more had a low-income cutoff of $31,822 and $32,554, respectively [34]. Therefore, since more than 80% of our sample had three persons or less per home, we dichotomized household income as "low income" (less than $39,999) and "middle/high" (more than $40,000). Whether a respondent had dental insurance was dichotomized as "yes" and "no. Finally, dwelling ownership was dichotomized in CCHS as "owned by member of household" and "rented".
Outcome. Oral health. Inadequate oral health was indicated as having at least one of the following oral health outcomes: bleeding gums, fair/poor perceived oral health and persistent oral pain. Bleeding gums was characterized in CCHS as the frequency of bleeding gums in the previous 12 months using the following four items: often; sometimes; rarely, and never. We further dichotomized bleeding gums as "present" (often, sometimes) and "not present" (rarely, never). Perceived oral health was measured in CCHS on a 5-point Likert scale from poor to excellent that asked the respondents to self-rate their oral health. We dichotomized perceived oral health as "fair/poor" (poor, fair) and "good" (good, very good, excellent). Finally, persistent oral pain was characterized as the frequency of persistent pain related to problems with the mouth in the previous 12 months using the following four items: often; sometimes; rarely; and never, and was further dichotomized as "present" (often, sometimes) and "not present" (rarely, never).
Covariates. We accounted for confounders that were selected a priori based on the literature suggesting these as potentially associated with both exposure and outcome. These included age (categorized as provided in CCHS into 30-44 years; 45-59 years and 60-74 years old,), biological sex (male, female), level of education (less than secondary school, secondary school and post-secondary education) and country of birth (Canada, other), as a proxy for immigration status.

Statistical analyses
First, we applied descriptive statistics to assess the characteristics of our study population using chi-squared tests. Next, we constructed multivariable binomial logistic regression models to estimate the prevalence ratios (PR) and 95% confidence intervals (CI) that quantify the association between perceived life stress (exposure) and inadequate oral health (binary outcome), adjusted for age, sex, level of education and country of birth.
We then followed the methods described by Knol and VanderWeele [35] to assess the effect measure modification (EMM) of indicators of social and economic capital on the association between perceived life stress and inadequate oral health, stratified by age group and adjusted for confounders. We estimated the relative excess risk due to interaction (RERI) as a standard measure for EMM on the additive scale. RERI corresponds to the risk that is in excess of what would be expected if the combination of psychosocial stress and social/economic capital was entirely additive. The RERI was calculated using the following equation: RERI = PR 11 -PR 10 -PR 01 + 1, where PR ab is the prevalence risk (PR) in the group with X 1 exposure status a (1 = exposed; 0 = unexposed) and X 2 exposure status b (1 = exposed; 0 = unexposed). The RERI was interpreted according to its direction, as opposed to its size. A positive value suggests that the combination of both exposures is higher than the sum of their independent effects on oral health.
The attributable proportion (AP) was also estimated. This measure was used to capture the proportion of the disease in the doubly exposed group that is due to the interaction between the two exposures [36]. The AP was calculated using the following equation: AP = RERI / PR 11 , where PR 11 is the prevalence risk in the group with exposure to both X 1 and X 2 . The Results of the EMM analyses are presented as proposed by Knol and VanderWeele [35]. All analyses were conducted using Stata/BE 17.0 (College Station, Texas).

Ethical considerations
As this was an analysis of publicly available data, we did not require approval from the Research Ethics Board at Western University, according to the Tricouncil policy statement for ethical conduct for research involving humans. No informed consent was required. Statistics Canada anonymized all registered data before the data were made available on their secured server.

Characteristics of study population
A total of 21,320 participants were included in the analysis of whom 44.9% (n = 9,584) were men, with a mean age (± standard deviation) of 51.2 (±10. 19). Characteristics of respondents included in this study are described in Table 1. Inadequate oral health (having at least one of the oral health outcomes: bleeding gums, fair/poor perceived oral health, and persistent oral pain) was equally distributed between both sexes and was most prevalent in the younger age group (30-44 years old, 43.8%). A greater proportion of participants who reported experiencing higher levels of perceived life stress also had inadequate oral health outcomes (47.0%) compared to those who reported low perceived life stress (32.7%). Men and women who reported experiencing higher levels of perceived life stress had higher prevalence of inadequate oral

Association between psychosocial stress and inadequate oral health outcomes
We used multivariable binomial logistic regression models to assess the association between perceived life stress and inadequate oral health. Our results showed individuals who reported having high levels of perceived life stress to be at a greater risk of experiencing inadequate oral health (PR = 1.39; 95% CI = 1.34, 1.44) than those who reported low perceived life stress.

Effect of social capital on the association of psychosocial stress with inadequate oral health across age groups
Living/family arrangements modified the association between perceived life stress and inadequate oral health across all three age groups, while sense of belonging to the local community only modified this relationship in adults aged 45-59 and 60-74 years old (Table 2). In younger adults (30-44 years old), low sense of belonging and high perceived life stress had a 1.57 (95% CI: 1.46, 1.69) higher risk of inadequate oral health compared to respondents with high sense of belonging and low perceived life stress (reference group); however, sense of belonging to the local community in this age group did not contribute to the stress-oral health relationship (RERI = -0.04). Younger adults who lived alone and have high perceived life stress also had a higher risk of inadequate oral health (PR = 1.33; 95% CI: 1.21, 1.47) and contributed to 6.0% of the association between stress and oral health (RERI = 0.08). Middle-aged adults (45-59 years old) who had high perceived stress and low sense of belonging to the local community (PR = 1.79; 95% CI: 1.66, 1.94) or who lived alone (PR = 1.54; 95% CI: 1.41, 1.68) continued to have a higher risk of inadequate oral health, and contributed to 12.3% (RERI = 0.22) and 7.8% (RERI = 0.12) of the perceived life stress and oral health association, respectively.
Both indicators of social capital had the largest magnitude of effect modification in older adults (60-74 years old). Older adults who had high perceived life stress and low sense of belonging to the local community (PR = 2.36; 95% CI: 2.06, 2.71) or high perceived life stress and who lived alone (PR = 2.10; 95% CI: 1.81, 2.44) had a higher risk of inadequate oral health and contributed to 30.1% (RERI = 0.71) and 12.5% (RERI = 0.25) of the association between perceived life stress and oral health, respectively. Attributable proportion: 0.08/1.33 = 6.0%

Sense of belonging to the local community
High sense of belonging

Effect of economic capital on the association of psychosocial stress with inadequate oral health across age groups
Indicators of economic capital also modified the association between psychosocial stress and inadequate oral health ( Table 3). The magnitude of effect was highest in older adults (60-74 years old) for all three indicators. Across all three age groups, respondents who had high life stress and low household income (<$40,000) had a higher risk of inadequate oral health. The magnitude of effect modification was highest among older adults (60-74 years old; PR = 2.33; 95% CI: 2.02, 2.69), contributing to 15.5% (RERI = 0.36) of the association between psychosocial stress and oral health. Household income contributed to 3.6% (RERI = 0.06) and 14.9% (RERI = 0.29) of the stress-oral health relationship in younger adults (30-44 years old) and middle-aged adults (45-59 years old), respectively.
Younger adults who had high perceived life stress and no dental insurance had a 1.45 (95% CI: 1.33, 1.58) higher risk of inadequate oral health compared to respondents with low perceived life stress who had dental insurance (reference group). Middle-aged (PR = 1.50; 95% CI: 1.35, 1.66) and older adults (PR = 2.03; 95% CI: 1.76, 2.33) also had a higher risk of inadequate oral health. Dental insurance had an attributable proportion of 4.1% (RERI = 0.06) and 4.4% (RERI = 0.09) for younger adults and older adults, respectively. Dental insurance did not contribute to the association between psychosocial stress and oral health in middle-aged adults (RERI = -0.11).

Discussion
In our study, we used methods of EMM to assess the extent to which indicators of social and economic capital modified the association between psychosocial stress and inadequate oral health in a large, provincially representative sample of adults from Ontario-Canada's most populated province. Our findings indicate that psychosocial stress, indicated by perceived life stress, is significantly associated with inadequate oral health, supporting previous studies with similar findings [7,9,37].
We also demonstrated that social and economic capital are modifiers of the psychosocial stress-oral health relationship, where better social and economic capital can be protective for oral health against effects of psychosocial stress, and that the magnitude of effect modification increased with age. It is well-known established that oral health declines with age [38]. For example, individuals aged 60 years and older have the highest incidence of dental caries, periodontal disease, oral cancer, and edentulism [39]. Economic capital has also been shown to decrease over time, where financial resources decline across the lifespan, particularly after retirement [40]. However, the literature is limited as to whether social capital follows a similar pattern. Social isolation and loneliness have been identified as risk factors for poor mental and physical health, particularly in older adults, and have been linked to reduced social networks, decreased economic resources, and changes in family structure and mobility [41]. Alternatively, other research suggests that older individuals experience an increase in neighbourhood socializing and volunteering, demonstrating flexibility and adaptability in maintaining friendship networks over time [42,43]. Our results suggest that social and economic capital may decrease with age, exacerbating the effects of psychosocial stress on oral health as individuals get older. Epidemiological theories of the social production of disease propose that social and economic positioning determine an individual's exposure to health-damaging stressors and allow for an improved access to various resources that aid in avoiding risks and mitigating the impact of the disease generation process [37]. For example, social capital at the individual level may affect oral health through social norms and informational social control, as well as through the diffusion of health-related knowledge through social networks [44,45]. Our findings are in support of previous research demonstrating that social capital protects against poor oral health outcomes. For example, one study found that in older adults, low social capital was a predictor of poor oral health-related quality of life [44]. Another study found adults reporting small social networks to be at a higher risk of having poor self-rated oral health [46]. Therefore, when individuals experience psychosocial stress, it is possible that low social capital, such as the lack of social networks or social support, can exacerbate the impact of psychosocial stress and contribute to the development of poor oral health. A biopsychosocial mechanism may also explain the modifying role of social capital. Poor social resources are postulated to increase stressful experiences that an individual might encounter as well as negatively impact health [47]. One way to characterize the biological response to chronic stress is by measuring allostatic load, which is a biological indicator of the "wear and tear" of physiological functioning resulting from the repeated and prolonged exposures to stressors [3,47,48]. Increased allostatic load has been demonstrated to increase the risks of several health conditions, including cardiovascular disease [48,49], depression [50,51], and periodontal disease [2,10]. This adds to the biological plausibility that the impact of psychosocial stress on oral health can be mitigated in individuals with a higher social capital, such as increased social networks or sense of belonging to the local community which in turn may ameliorate the stressful exposures and curb the biological stress toll [47].
Previous research has demonstrated that the relationship between current stress and poor oral health may also be moderated by indicators of economic capital such as socioeconomic position and the availability of dental insurance [52]. Low economic capital may exacerbate a multitude of health barriers related to oral health. For example, individuals with low economic capital may experience: (1) worse access to adequate dental care due to unemployment or employment in a position with poor health benefits [53]; (2) greater chronic stress and stresscoping behaviours that are more damaging to oral health (e.g., tobacco use, alcohol use, sugar consumption) [52]; and/or (3) greater risk of other chronic diseases, which may impact oral disease via the oral-systemic health link [54][55][56]. Our findings that indicators of economic capital, such as household income, dental insurance, and dwelling ownership, modify the association between psychosocial stress and oral health support the hypothesis that individuals with higher economic capital can mitigate the possible impacts of psychosocial stress on poor oral health.
The findings of our study further highlight the need for initiatives that address the social determinants of health, particularly ones that tackle elements of social and economic capital in order to mitigate the harmful impact of stress. For example, strategies can be suggested to support community and local environments that enhance an individual's sense of belonging and engagement in their community to promote their oral and general health. Similarly, policies that strengthen economic capital such as those directed towards enhancing income and housing equality and universal access to dental care can also protect against the negative impacts of psychosocial stress on oral health [26,57,58].

Strengths and limitations
Some of the strengths of our study include our use of a large, provincially representative sample of adults aged 30-74 years old. Also, we used multiple indicators to generate a construct variable of oral health. We also used multiple indicators for each of our social and economic capital variables to ensure that our results are not sensitive to a specific measure, but rather applicable to various exposure variables. Our study also has a few limitations, including its reliance on self-reported measures of perceived life stress, social capital, economic capital, and oral health, which may make the responses subject to recall and social desirability biases [59]. Our study is also limited by the cross-sectional nature of CCHS data and subsequently our cross-sectional study design, which limits our ability to make inferences about temporal sequence or causation. In order to better understand the mechanisms linking psychosocial stress to oral health, and to gain further insights into the roles of social and economic capital in this relationship, longitudinal study designs and/or causal inference analyses can be employed in future studies.

Conclusion
Overall, our findings corroborate the notion that perceived life stress increases the risk of inadequate oral health. We demonstrate that this relationship is buffered by enhanced social and economic capital. Within its limits, our study warrants greater attention to the role of psychosocial stress in oral diseases, and importantly, necessitates action on the social and economic factors that can play a role in mitigating the impact of stress on oral health. Future work should examine possible social, psychosocial, and biological mechanisms linking social and economic capital to inadequate oral health.